TEMPERATE GRASSLAND RESPONSES TO CLIMATE-CHANGE - AN ANALYSIS USING THE HURLEY PASTURE MODEL

The Hurley Pasture Model is process-based and couples the carbon, nitr ogen and water cycles in the soil-grass-animal system. It was used to examine the responses of grasslands in southern, lowland and northern, upland climates in Britain. Short-term response to step-wise increase s in CO2 concentration (350 to 700 mu mol mol(-1)) and temperature (5 degrees C) were contrasted with long-term equilibrium (the term 'equil ibrium' is equivalent to 'steady state' throughout this paper) respons es and with responses to gradually increasing [CO2] and temperature. E quilibrium responses to a range of climate variables were also examine d. Three conclusions were drawn regarding the interpretation of experi ments: (1) initial ecosystem responses to stepwise changes can be diff erent in both magnitude and sign to equilibrium responses, and this ca n continue for many years; (2) grazing can drastically alter the magni tude and sign of the response of grasslands to climate change, be high ly site-specific. It was concluded that experiments should try to less en uncertainty about processes within models rather than try to predic t ecosystem responses directly. Three conclusions were also drawn abou t the operation of grasslands as carbon sinks: (1) increasing [CO2] al one will produce a carbon sink, as long as it continues to accelerate photosynthesis and increase net primary productivity; (2) by contrast, increasing temperatures alone are likely to produce a carbon source, because soil respiration is accelerated more than net primary producti vity, even when assuming the same temperature function for most soil a nd plant biochemical processes; and (3) the net effect of projected in creases in [CO2] and temperature is likely to be a carbon sink of 5-15 g C m(-2) yr(-1) in humid, temperate grasslands for several decades, which is consistent with the magnitude of the hypothesized current glo bal terrestrial carbon sink. (C) 1997 Annals of Botany Company.